Shouldn't we also account for the growing prevalence of share buybacks?

Nope.
Since most buybacks aren't on average done at valuations below true value, they don't add value to that stock or to the index.

On average a stock isn't worth any more the day after a buyback than it is the day before. The earnings per share go up, but the cash per share goes down.
Some people look at the first one and totally forget about the second one.
Since it's true for the average stock, it's true for the set of all stocks.

There are some things that truly change the value of companies and the trend line of that value.
For example, the huge rise in US net corporate profits margins in the last 20-40 years is a true boon.
But that's baked into the index valuation method I use, which is the series of recent real profits, just smoothed.
There is a bit of lag, but US net margins have been high for quite a while now. Since 2006, except for the short sharp dip during the credit crunch.
Other than the credit crunch bungee, they haven't even dipped as low as the very highest level seen in the stretch 1951-2004.
More of GDP to net corporate profits, partly because a lower share has been going to employees. Also lower tax and lower real interest costs.

A huge broadly-based increase in corporate leverage will also boost profits, for a while at least.
The average tends to get pulled down again later on when some of the bigger companies go to zero from the added brittleness.

There are also things that don't actually add to true value, but change the amount apparent value because of higher reported net profits.
e.g., the small rise in apparent profits arising for the cancellation of the requirement to amortize all goodwill from acquisitions.

But buybacks aren't a factor.

Jim